In a pumping system for highly compressible fluids, an important feature is the ability to keep the pump head of the pumping apparatus and the pumping fluid at a very low temperature to reduce the compressibility of the pumping fluid prior to pumping. Additionally, it is an important feature to stabilize fluid flow and to reduce flow ripple by maintaining the pumping fluid at a constant temperature. Currently, there is intense interest in using near-critical and supercritical fluids at elevated pressures as solvents in extraction systems and in chromatographic systems. Often the solvents of interest exist as highly compressible fluids at ambient pressures and temperatures from 15-40 degrees centigrade. At ambient conditions, fluids such as carbon dioxide, ethylene, ethane and sulfur hexafluoride have high vapor pressures which significantly exceed 1 atmosphere. However, those pressures are not sufficiently high for extraction and chromatographic applications at or near supercritical conditions. Therefore, gaseous or liquid state fluids must be supplied to some type of pumping system to meet pressure and flow requirements of the high pressure processes downstream of the pumping system.
Compressing and injecting compressible fluids into a high pressure system at mass flow rates in the range of 0.2 g/min to 10 g/min (for CO.sub.2) is very difficult. A multiple compressor pump system may be used for this purpose or the gas may be condensed by removing the heat of vaporization. Once the fluid in the gaseous phase is liquified, the fluid may be introduced into the pumping system to ensure constant mass flow rates. Since extraction or chromatographic compressible solvent fluids are typically supplied in the liquid phase from a pressurized tank, a pressure drop generally arises prior to entering the pump. Flashing may occur where fluid in the liquid phase could then be mixed with fluid in the gas phase resulting in a two phase fluid mixture which is more compressible than the original signal phase fluid. Therefore, the efficiency and metering accuracy of a solvent delivery system can be greatly enhanced by decreasing the compressibility and hence increasing the bulk modulus of the fluid by precisely maintaining the pumping fluid at sub-ambient conditions. Another alternative would be to increase the compression ratio of the pump.
Current solvent delivery systems utilize syringe pumps with pumping cylinders having large compression ratios. A major drawback of such systems is the need to interrupt the chromatographic process to refill the cylinder once it is empty. Current reciprocating, diaphragm pumps or compressors require large compression ratios and are relatively large in implementation.
In order to decrease the compressibility of the fluid and thereby decrease the corresponding compression ratio so that liquid-type pumps with continuous flow capability can be used, it is necessary to pre-cool the compressible fluid prior to entry into the pump. Heat exchangers and cooling baths are typically employed for this purpose. Furthermore, it is also necessary to cool the pump head separately to keep the compressibility of the fluid constant at a low value during the pumping process. However, this typically requires the use of a complicated system incorporating two thermal cooling zones.
Pre-cooling of the pumping fluid 1 to sub-ambient temperatures has been accomplished by feeding the pumping fluid through a heat exchanger 2 of a pre-cooler 4 placed in a recirculating bath 5 containing a cooled liquid having a temperature regulated by a thermocouple 6 (see FIG. 1). The cooled liquid is also circulated to the pump head 7 by a recirculating pump 8 to keep it at sub-ambient temperatures. However, this approach requires a lot of equipment and makes it difficult to accurately regulate the temperature of both the pumphead and the pre-cooler at the same time. Another problem associated with this apparatus is maintaining constant temperature of the pumping fluid once it leaves the precooler 4 and flows to the pumphead. The plumbing in between is very insulated, however, the fluid still cannot maintain constant temperature because of significant thermal interaction with the environment. The same problem is true with respect to the recirculating cooling fluid for the pumphead. Since the cooling source is remote from the pumphead, thermal inefficiencies are encountered. FIG. 2 is a more detailed view of the pumphead and illustrates problems associated with cooling the pumphead and the pumping fluid separately.
It is, therefore, desirable to eliminate the need for two controlled zones of cooling such as a recirculating bath or cryogenic cooling.
In particular, it would be desirable if the need for a recirculating bath could be eliminated completely.